CT scanning systems and methods are well known in the art, particularly for medical imaging and diagnosis, but also in other field of imaging, for example, industrial quality control.
CT scanners generally create images of one or more sectional slices through a subject's body. A radiation source, such as an X-ray tube, irradiates the body from one side thereof. A collimator, generally adjacent to the X-ray source, limits the angular extent of the X-ray beam, so that radiation impinging on the body is substantially confined to a planar region defining a cross-sectional slice of the body. At least one detector (and generally many more than one detector) on the opposite side of the body receives radiation transmitted through the body substantially in the plane of the slice. The attenuation of the radiation that has passed through the body is measured by processing electrical signals received from the detector.
Typically, in commonly-used third- and fourth-generation CT scanners, the X-ray source (or multiple sources) is mounted on a gantry, which revolves about a long axis of the body. In third-generation scanners, the detectors are likewise mounted on the gantry, opposite the X-ray source, while in fourth-generation scanners, the detectors are arranged in a fixed ring around the body. Either the gantry translates in a direction parallel to the long axis, or the body is translated relative to the gantry. By appropriately rotating the gantry and translating the gantry or the subject, a plurality of views may be acquired, each such view comprising attenuation measurements made at a different angular and/or axial position of the source. Commonly, the combination of translation and rotation of the gantry relative to the body is such that the X-ray source traverses a spiral or helical trajectory with respect to the body. The multiple views are then used to reconstruct a CT image showing the internal structure of the slice or of multiple such slices, using methods known in the art.
The lateral resolution of the CT image, or specifically, the thickness of the slices making up the image, is generally determined by the angular extent of the radiation beam or of the individual detectors, whichever is smaller. The use of thick slices is advantageous in increasing the signal/noise ratio, and thereby reducing the time needed to acquire the data needed to reconstruct an image. But images reconstructed using thick slices have poor resolution in the axial direction and are more susceptible to partial volume artifacts, i.e., imaging errors that are introduced when a single volume element (voxel) within a slice contains two types of tissue having different attenuation coefficients.
Smaller detectors are generally used, therefore, to improve axial resolution and reduce partial volume artifacts. Excessive reduction of the extent of the detector, however, leads to degradation of the signal/noise ratio and decreases the throughput of the imaging system. Using very small detectors can also reduce the system's dose efficiency, i.e., increase the relative amount of radiation to which the portion of the body being imaged is exposed, because the angular extent of the X-ray beam irradiating the body will typically extend somewhat beyond the bounds of the detectors. Radiation outside these bounds is "wasted," since it is not used in forming the CT image.
In order to improve throughput, as well as increase axial resolution and utilize the X-ray source more efficiently, various inventors have described the use of differently configured detector arrays. Such arrays typically include a plurality of radiation detectors, such as scintillator-photodiodes, which receive radiation simultaneously from a radiation source and are thereby used to acquire multiple views and/or multiple slices simultaneously. Spiral modes of translation and rotation, as mentioned above, are frequently combined with multi-slice image acquisition to cover a greater volume of the body in less time with improved axial resolution.
For example, U.S. Pat. No. 4,965,726, to Heuscher, et al., whose disclosure is incorporated herein by reference, described a CT scanner with a plurality of segmented detector arrays. Each array includes a plurality of rows of radiation-sensitive cells. The rows may have different dimensions in a lateral direction, perpendicular to the long dimension of the rows, and the effective lateral dimensions of the rows may be varied by moving collimators adjacent thereto, so as to provide slices of the same or different lateral thicknesses. Multiple detectors may be grouped together in the lateral direction to provide thicker slices, so as to improve the signal/noise ratio and throughput of the scanner, while reducing partial volume artifacts relative to slices of comparable thickness that are acquired using a single detector having an equivalent lateral dimension.
U.S. Pat. No. 5,241,576, to Lonn, whose disclosure is likewise incorporated herein by reference, similarly describes a CT scanner including an array of detector elements for the purpose of acquiring thick-slice images with reduced partial volume artifacts. The array includes a plurality of detector elements, wherein each such element includes a set of sub-elements disposed along the slice thickness (lateral) dimension. The signal output of each sub-element is processed individually, generally by taking the log of the signal and applying a weighting factor thereto. The processed outputs of the plurality of sub-elements belonging to a single element are then summed together to form a combined thick-slice signal.
U.S. Pat. No. 5,430,784 to Ribner, et al., whose disclosure is also incorporated herein by reference, describes a CT scanner and detector array having a plurality of rows of identical detectors, which are connected together by a controllable switching matrix. This switching matrix is controlled to interconnect a predetermined number of successive detector sub-elements, in order to produce combined signals corresponding to one or more slices of a desired thickness.
U.S. Pat. No. 4,417,354, to Pfeiler, whose disclosure is incorporated herein by reference, describes a CT scanner including a detector array that is mounted to pivot about a lateral axis, perpendicular to an image slice acquired by the array. The array is pivoted in order to increase the effective resolution within the image slice, but only a single image slice is provided, and no suggestion is made of changing the slice thickness by pivoting the array about a transverse axis.
Similarly, U.S. Pat. No. 5,493,593, to Muller et al., whose disclosure is incorporated herein by reference, describes a scanner for CT microscopy including a tiltable detector array, which is also shifted horizontally in order to maximize the utilization of the array. Only a single image slice is provided, however, without suggestion of changing the slice thickness.